The first time in roughly four decades that a privately developed, non‑light‑water advanced reactor has gone critical in the U.S.
Posted by Leslie Eastman
The quiet revolution in American energy just cleared a major technical hurdle.
At Idaho National Laboratory, Antares Nuclear’s Mark-0 microreactor achieved zero-power criticality under the Department of Energy (DOE) Reactor Pilot Program (RPP), becoming the first advanced reactor to meet the ambitious July 4, 2026, deadline set by President Trump’s 2025 executive order to accelerate next-generation nuclear deployment.
The U.S. energy secretary called the development “one of the most significant achievements in nuclear energy in forty years.”
The Department of Energy said the Mark-Zero reactor design by Antares Nuclear, hosted at Idaho National Laboratory, achieved criticality. That means the team started a self-sustaining chain reaction of nuclear fission, the fundamental process for operating a reactor.
The Department of Energy said it is the first time a privately developed, non-light-water reactor has gone critical in four decades, and that it serves as a blueprint for other advanced reactors supported by the Trump administration.
State leaders praised the accomplishment. Sen. Jim Risch posted on social media that Idaho is “leading the charge” to a new era of advanced reactors.
WE HAVE CRITICALITY! On June 4, 2026, at around 12:30 MDT, @AntaresNuclear’s Mark-0 microreactor achieved initial criticality at @INL.
It is the first nuclear test reactor to go critical under @ENERGY’s Reactor Pilot Program. Congratulations to Antares on reaching this… pic.twitter.com/JS9g9Vsm26
— Office of Nuclear Energy | US Department of Energy (@GovNuclear) June 4, 2026
The zero‑power test was designed to confirm that the core’s reactivity, control systems, and instrumentation behave as predicted, before any attempt at higher‑power operation. The next step in the plan is to get the system to generate sustained electricity, and that phase is also scheduled for this year.
“Now that Mark-0 is critical, the real work is just beginning,” said Antares CEO Jordan Bramble in a LinkedIn post on June 4. “I want to reiterate how this fits into our larger roadmap to mature our technology to its commercial potential. This should be obvious, but the goal of a reactor is to sell electricity to customers.”
Following reactor physics experiments, Antares will execute “the next phase of our roadmap—sustained electricity production,” Bramble said. Antares is “able to move fast towards this milestone because we’ve already completed over 6 months of full-power thermal testing in an electrical prototype. We will perform version 2.0 of this in 2026. This is an easier, more iterative way to test, because there is no regulatory process, and you can disassemble to examine material effects.”
He added: “All of our iterative testing sets us up to produce electricity for 6+ months. Hundreds of days, not hundreds of hours. We’re able to test for longer and faster because we’ve designed our reactor around a proven, fully qualified fuel spec developed under Project Pele.”
Some of the great Americans who made Mark-0 possible. Including @Waksman84 @colejust12 @JessGehin @OverReactor1776
Jeff’s work on project Pele began 8 years ago. The program trained operators, improved the regulatory process, and created the fuel line that made our TRISO. pic.twitter.com/cjpJcGqrKN
— Jordan Bramble (@jordanbramble) June 6, 2026
Presidential Executive Order 14301, “Reforming Nuclear Reactor Testing at the Department of Energy,” the RPP also supports the Army’s Janus Program, which focuses on deploying advanced nuclear microreactors for operational and installation energy needs.
The Janus Program is the Army’s answer to Presidential Executive Order 14299, “Deploying Advanced Nuclear Reactor Technologies for National Security.” The Army coordinated with the Energy Department to ensure fuel was fabricated in time to support the criticality timeline and also provided expertise to support the safety review of the Antares criticality test.
“This is a historic moment for advanced nuclear energy in the United States,” Energy Secretary Chris Wright said. “The Antares team has achieved a major milestone in the development of micro-reactor technology, and the Trump administration is proud to have supported their efforts. These technologies will play a critical role in strengthening U.S. energy security for both military and civilian applications.”
Jeff Waksman, principal deputy assistant secretary of the Army for installations, energy and environment, said, “The criticality test of Antares Nuclear’s Mark-0 reactor is an important step toward meeting the president’s goals for nuclear power resiliency for the U.S. Army. A microreactor is now generating neutrons.”
When America sets bold goals, America delivers.
Last year, President Trump challenged our nation to bring multiple advanced reactors to criticality by America’s 250th Anniversary. Today, we reach an important milestone as the first non-light-water reactor goes critical in more… https://t.co/XWuuEF92Dy
— Secretary Chris Wright (@SecretaryWright) June 4, 2026
This milestone, while not yet producing electricity, confirms the reactor’s core physics and safety behavior in the real world, not just in modeling, and sets the stage for operational microreactors expected as early as 2027.
In an energy landscape increasingly defined by grid instability and rising demand, that’s not just progress, but a signal that U.S. nuclear innovation may finally be moving at the speed policymakers have been demanding.
